Self-organization on social media: endo-exo bursts and baseline fluctuations

A salient dynamic property of social media is bursting behavior. In this paper, we study bursting behavior in terms of the temporal relation between a preceding baseline fluctuation and the successive burst response using a frequency time series of 3,000 keywords on Twitter. We found that there is a fluctuation threshold up to which the burst size increases as the fluctuation increases and that above the threshold, there appears a variety of burst sizes. We call this threshold the critical threshold. Investigating this threshold in relation to endogenous bursts and exogenous bursts based on peak ratio and burst size reveals that the bursts below this threshold are endogenously caused and above this threshold, exogenous bursts emerge. Analysis of the 3,000 keywords shows that all the nouns have both endogenous and exogenous origins of bursts and that each keyword has a critical threshold in the baseline fluctuation value to distinguish between the two. Having a threshold for an input value for activating the system implies that Twitter is an excitable medium. These findings are useful for characterizing how excitable a keyword is on Twitter and could be used, for example, to predict the response to particular information on social media.Comment: Presented at WebAL-1: Workshop on Artificial Life and the Web 2014 (arXiv:1406.2507

The Concentration-Density Relation of Galaxies in Las Campanas Redshift Survey

We report the results of the evaluation of the ``concentration-density'' relation of galaxies in the local universe, taking advantage of the very large and homogeneous data set available from the Las Campanas Redshift Survey (Shectman et al. 1996). This data set consists of galaxies inhabiting the entire range of galactic environments, from the sparsest field to the densest clusters, thus allowing us to study environmental variations without combining multiple data sets with inhomogeneous characteristics. Concentration is quantified by the automatically-measured concentration index $C$, which is a good measure of a galaxy's bulge-to-disk ratio. The environment of the sample galaxies is characterized both by the three-space local galaxy density and by membership in groups and clusters. We find that the distribution of C in galaxy populations varies both with local density and with cluster/group membership: the fraction of centrally-concentrated galaxies increases with local galaxy density, and is higher in clusters than in the field. A comparison of the concentration-local density relation in clusters and the field shows that the two connect rather smoothly at the intermediate density regime, implying that the apparent cluster/field difference is only a manifestation of the variation with the local density. We conclude that the structure of galaxies is predominantly influenced by the local density and not by the broader environments characterized by cluster/field memberships.Comment: 11 pages, 4 figures, ApJ in press, uses psfig.st

Magnetization dependent current rectification in (Ga,Mn)As magnetic tunnel junctions

We have found that the current rectification effect in triple layer (double barrier) (Ga,Mn)As magnetic tunnel junctions strongly depends on the magnetization alignment. The direction as well as the amplitude of the rectification changes with the alignment, which can be switched by bi-directional spin-injection with very small threshold currents. A possible origin of the rectification is energy dependence of the density of states around the Fermi level. Tunneling density of states in (Ga,Mn)As shows characteristic dip around zero-bias indicating formation of correlation gap, the asymmetry of which would be a potential source of the energy dependent density of states